Crab's body is divided into head, chest and abdomen. The back of the head and chest is covered with head and chest armor, and the shape varies from species to species. There are 1 and 2 pairs of antennae in the center of the forehead, and compound eyes with stalks on the outside. The mouthparts include 1 pair of jaws, 2 pairs of jaws and 3 pairs of jaws. There are five pairs of chest feet on both sides of the head breastplate.

The abdomen is degenerated, flat and tortuous on the ventral surface of the head and chest. Male abdomen is narrow and long, mostly triangular, and only the first two pairs of appendages are deformed into connectors; The female has a broad abdomen, with 1 pair of double-branched appendages in the 2nd to 5th segments, which are densely covered with bristles for holding eggs. Most crabs are marine, with the most species in tropical shallow waters.

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During crab culture, some diseases seriously affect the benefit of the farm. For example, the prevalence rate of crustacean canker is extremely high in the local area, with typical symptoms: at the initial stage of the epidemic, the crab shell has many brown spots with a reddish depression in the middle. In the later period of epidemic, the spots became black, the symptoms of ulcer intensified, and they began to erode, even the body of crabs was clearly visible.

The epidemic rate of this disease is extremely high in the local area, which is harmful and affects the final breeding benefit. The best way of treatment, on the first day, use 10% povidone iodine, use 250mL/667m2, mix with water and sow. The next day, use 30% vitamin C, 250g/667m2, mix with water and sow. The harm of this disease affects the water quality. Daily prevention and control, disinfection should be done well, and water quality management should be strengthened.

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